Traditional printing uses a colorimetric approach to visually match a printed color to a color of source content. Colorimetry approximates human color perception by representing observed colors by their tristimulus values—one for each of the three types of light sensitive cells (cones) lining the retina at the back of the human eye. One type of cone cell is sensitive mostly to long wavelengths (L), another to medium wavelengths (M) and yet another to short wavelengths (S) of electromagnetic radiation in the visible range (i.e., from approximately 400 to 700 nm). Mostly for historical reasons, colorimetry uses a linear transformation of the LMS space, derived from psychophysical color matching experiments, called XYZ, defined by the CIE (Commission Internationale de l'Eclairage). The tristimulus values of X, Y and Z of a colorimetric approach form the basis of approximately representing all colors seen by a human visual system. Colors are thus identified based on co-ordinates in this CIE XYZ space.
Colorimetric reproduction, for example using the CIE XYZ color space, is generally limited to a set of viewing conditions such as a standard observer and a single illuminant. A standard observer is one that represents the average human's response to light falling on the receptors of the eye. Illumination conditions may change depending on the time of day for which observation is taking place, for example sunset gives rise to an illuminant of lower Correlated Color Temperature (CCT) falling on objects as opposed to an illuminant of higher CCT at midday. Therefore, a surface viewed under one illuminant will not give the same observed color when viewed under a second illuminant. Both the standard observer and illuminant are considerations for color reproduction and for performing visual matching. Traditionally, the standard observer and standard illuminant (e.g. D50) are used to visually match colors observed between an original and a reproduction. However, when the observer or the illumination conditions are changed, the reproduction may no longer be accurate and appear different to the original.
A gamut provides a representation of available colors in a color space, for example a CIE color space. The output of an imaging system, for example a printer, display device or projector, is judged in terms of its attributes such as smoothness, noise or grain, color constancy under different viewing conditions and the use of resources, such as ink for printers and power for projectors and display devices, for delivering that output. For many attributes the performance of the imaging system is worse in some parts of the color gamut than in others. In for example a printing system, colors are represented by a combination of inks of different colors deposited on a substrate and it is desirable to limit the amount of ink used to represent a color.